1. Field of the Invention:
The invention in general relates to side-looking sonar systems, and particularly to a system which provides for ultra-high resolution as well as multibeam capability.
2. Description of the Prior Art:
In side-looking sonar systems, a transducer on a carrier vehicle periodically projects pulses of acoustic energy, athwartship of carrier travel toward a target area such as the sea bottom, as the carrier proceeds along a course line. The area upon which the acoustic energy impinges is known as the insonified area and acoustic energy reflected back from the insonified area, as well as targets on it, is received by a receiver transducer. The receiver beam associated with the receiver transducer is extremely narrow in the direction of carrier travel such that the receiver detects reflected acoustic energy from relatively narrow insonified strips (receiver strips) on the sea bottom, during the course of travel.
The received acoustic energy is processed and displayed on a suitable display apparatus. With each pulse transmission and subsequent reception, a scan line is produced on the display to build up a picture of the sea bottom in a manner similar to the scanning of a conventional cathode ray beam in a television picture tube with the presentation being a pattern of highlights and shadows with objects outlined in such a way as to permit their identification.
For increased resolution there has been developed a side-looking sonar transducer which is of a curved configuration wherein the transducer segments forming the receiver transducer lie along an arc of a circle whose radius is the design altitude and wherein acoustic energy is in focus along a line of focus on the sea bottom. These focused transducers must be used at a precise altitude above the sea bottom (plus or minus some limited depth of field) since excursions above or below the design altitude tend to defocus the operation outside of the depth of field and consequently degrade the display to a point where proper target identification is severely limited.
A side-looking sonar system has been developed which provides for relatively high resolution operation without the requirement for operating at a single precise altitude. The system, described in U.S. Pat. No. 3,950,723, hereby incorporated by reference, basically utilizes a multi-segment straight line transducer with signal processng channels connected to each segment of the transducer to continuously and electronically vary the focus as a function of time. The effect of this operation is in essence to simulate a transducer with a sharp curvature to focus close-in early after a transmitted pulse then to decrease the curvature with time as the return comes in from longer ranges. High resolution is achieved with a substantially infinite depth of field which is not necessarily desirable since surface reflections would show up in some instances where objects on the bottom are searched.
The resolution obtainable with such systems is an inverse function of transducer length, and according for ultra-high resolution work the transducer is extremely long, even exceeding 1000 .lambda. where .lambda. is the operating wavelength. With an increased transducer length the number of segments making up the transducer would increase thereby necessitating a significant increase in the number of signal processing channels connected to the transducer segments.
In response to impingement upon a straight line transducer of acoustic energy having a curved wave front, a certain phase differential is experienced by an individual segment from one end to its other. The length of the segment, and accordingly the total number of segments required for a particular length transducer, is determined by a maximum allowable phase differential.
If a curved transducer could be used in place of the straight line transducer, the curvature would more closely match the curved wave front of the acoustic energy received and there would be less of a phase differential from one end of a segment to its other such that greater length segments may be utilized in making up the total length of the transducer and consequently fewer signal processing channels and less electronic circuitry would be required. However, with the extremely long arc transducer for ultra-high resolution operation, the depth of field would be so small as to make the system impractical over a rugged terrain where the distance from the carrier vehicle to the bottom would be subject to frequent variations greater than the depth of field capability of the system.
The present invention obviates the limitations of prior art systems by allowing the use of extremely long arc transducers for obtaining ultra-high resolution displays of a target area and does so without the requirement for flying at a precisely controlled altitude over the target area.
Since the receiver strip upon the target area is extremely narrow in the direction of carrier travel, for ultra-high resolution work, it is desirable that more than one receiver beam be formed in the time it takes for a single acoustic pulse to travel out to the maximum range and return. A multi-beam capability with an arc transducer is described in U.S. Pat. No. 3,742,436, however, the receiver beams, and therefore the receiver strips are not parallel to one another but extend generally radially from a point tending to leave objectionable gaps in the display whereby targets may be missed. The patent does describe the generation of a multiplicity of parallel beams, however, a plurality of arc transducers, each operating at a different frequency, is required thus adding to the cost and complexity of the system. The present invention further allows for the formation of a limited number of additional beams whereby the receiver strips on a target area are generally parallel to one another.